eso9834 — Photo Release

The VLT Records Faint Structures in Colliding Galaxies

31 July 1998

The unsual galaxy IC 1182 is located in the Hercules cluster of galaxies , a large assembly of galaxies that is seen in the constellation of that name, somewhat north of the celestial equator. It is one of the principal members of this cluster to which the measured distance is of the order of 500 million light-years.

The first VLT 8.2-m telescope (UT1) has recently obtained some spectacular images of this complex galaxy, showing features that have never been seen in this detail before. This has been possible, due to the large light-gathering capacity and excellent optical quality of this telescope. In fact, the UT1 is particularly well suited to (detect and) image very faint, extended structures (low surface brightness features), also in distant objects.

The present photos show two views of IC 1182. They are based on a mosaic of sky exposures with the VLT Test Camera through a red optical filter. The first (ESO Press Photo eso9834a, to the left) shows the entire field that was observed. It is reproduced in "negative" (black stars on a white sky) in order to show the faint outer structures. The second (ESO Press Photo eso9834b, right, in "positive") displays the intricate features in the central area in more detail.

This galaxy possesses a spectacular jet that extends from the centre in the East direction. It is composed of a series of knots of energetic material that move outward from the center. Moreover, there is a broad, but fainter, S-shaped plume on the other side, as well as other "debris" all around. (The circular structure near the right edge of this plume is an instrumental artefact).

The central region of IC 1182 has strongly emitting gas and many clusters of hot, blue stars. Together with the other features, they indicate the very "active" nature of this galaxy. It radiates a lot of energy and ejects a substantial amount of material into the surrounding space at high velocities.

There have been various attempts of explaining this activity, but the most likely one is that of a collision between two galaxies . This view is supported by the observation of two galaxy centers (astronomers refer to them as "galaxy nuclei" ) near each other. This is well visible on ESO Press Photo eso9834b, where the second nucleus is quite obvious, just below to the right (at "4 o'clock") of the main one.

It appears that the structures seen on the VLT images have originated in a collision between a large, elliptical galaxy and another, very gas-rich one, possibly a spiral galaxy. The latter has now been almost completely disrupted by strong gravitational forces, leaving a large amount of gas that is now observed in the extended structures of IC 1182.

More information

Technical information : Combination of six 10-min R (red) exposures (seeing 0.6 - 0.7 arcsec) obtained on June 24, 1998, with the VLT Test Camera at three offset pointings. Individual frames were flat-fielded and combined to produce a final FWHM = 0.63 arcsec. The field shown measures 2.4 x 2.1 arcmin (eso9834a) and 1.2 x 1.1 arcmin (eso9834b), respectively. Assuming a distance of 500 million light-years (i.e. that of the Hercules cluster), this corresponds to approx. 350,000 and 175,000 light-years, respectively. North is to the upper right; East is to the upper left.